I have a problem. I mosaiced (image processing/restoration/mosaic) four different Landsat images into one. But now I am not sure how to equalize the brightness and contrast of the whole picture, because some images are brighter than the rest, and I am afraid that I will get the wrong results after making color composite, classifying data and so on... I am new to Gis and I would really appreciate it if someone helped me.

I'm not sure about IDRISI, but I know you can easily refine the brightness and contrast of images in Geomatica, an image processing product by PCI Geomatics. It's actually very easy to color balance 4 landsat images in Geomatica.

You can download a trial of Geomatica on their website at http://www.pcigeomatics.com/resources/more-resources/downloads.

I suggest you watch this mosaicking webinar that demosntrates how to fix brightness and contrast, http://www.pcigeomatics.com/ortho-mosaicking-with-geomatica

You will get odd results using the mosaic you describe. It is usually best (and simpler) to match the contrast of your images before or as you mosaic. Most professional mosaic tools have contrast matching options so that the output mosaic does not look like a patchwork quilt.

Does Idrisi have contrast matching options as you mosaic? If not, does it have tools to match separate images?